Montana Governor's Office of Economic Development http://business.mt.gov/ should be your first stop.
Business Registration: Register online with the Montana Secretary of State https://www.montanasos.com/ , or find the forms you need via their site.
Business Permits & Licenses: The Montana Department of Labor & Industry http://bsd.dli.mt.gov/ has information about licenses, as do the Montana government's official site http://revenue.mt.gov/licenses and the Montana Small Business Development Center https://sbdc.mt.gov/Resources/Licensing#One-Stop-Licensing-525 .
Tax Registration: The Montana Department of Revenue https://revenue.mt.gov/home/businesses has the tax forms you need.
Local Permits & Licenses: Directories for Montana county offices and city offices http://www.mtcounties.org/counties/county-information/montana-counties-contact-information are available.
By Alexander Huls
Full Story: https://sba.thehartford.com/finance/how-to-start-a-small-business-in-all-50-states?cmp=EMC-SC-SBA-07199486